1. Field of the Invention
The present invention relates to a vibration driven actuator apparatus and a vibration control method therefor.
2. Description of the Related Art
Vibration-wave driven actuators have strain generating elements (e.g., electro-mechanical energy conversion elements) which cause mechanical strain in response to an applied electric field or magnetic field. These strain generating elements are mounted in an elastic member which functions as a vibration member. The vibration member is vibrated by the strain generating elements, so that a vibration of the vibration member is converted into continuous or intermittent mechanical movements, which can then be output. Such actuators have recently been used in various fields. Conventionally, a piezoelectric/electrostrictive actuator using a piezoelectric element or electrostrictive element as the strain generating element is most widely used. Since a piezoelectric actuator using a piezoelectric element, also called an ultrasonic-wave motor, is capable of forming a continuous rotary type drive source, it has already been utilized in an optical apparatus, such as a camera, as a drive source in place of a conventional rotary electromagnetic drive motor, and the technology for driving and controlling such an ultrasonic-wave motor generally is known.
The drive control technology for ultrasonic-wave actuators generally has been established over the past several years (The description in this specification is concerned with ultrasonic-wave actuators or ultrasonic-wave motors of the type in which mechanical resonance, which occurs in the vibration member when an alternating electric field or magnetic field is applied to the strain generating element, is made into an output displacement). However, drive control technology for controlling a start-up operation of the actuator and a transient drive state need to be further improved.
In conventional ultrasonic-wave motors installed in an optical apparatus such as a camera, it is well known that, since a moving member serving as a rotor is strongly pressed against a vibration member (stator) by means of a spring, when a stationary moving member is initially started, a force larger than the sum of the stationary friction force between the vibration member and the moving member and the stationary inertial force of the moving member must be applied to the moving member. Therefore, in the conventional drive control technology, control is effected in such a way that a voltage greater than an alternating voltage applied to the piezoelectric element during a normal operation is applied to the element at the start time. However, since such a motor is not always reliably started by this conventional control method, an alternative drive control technology has been proposed, for example, in U.S. Pat. No. 4,560,263, in which standing wave vibrations are generated in the vibration member before starting so that the sticking force between the vibration member and the moving member is weakened before the motor is started (the moving member does not move because traveling waves are not generated).
However, according to an experiment conducted by the inventors of the present invention regarding the ultrasonic-wave motor, it has been found that, although in the drive control technology disclosed in the above publication the transition time from the stationary state to the normal operating state (the state in which traveling wave vibration is generated in the vibration member) can be shortened, reliable starting of the motor is not easily accomplished even at a low voltage. When the inventors of the present invention conducted various experiments as to the cause of this drawback, it was found that when the motor could be started, a close-contact phenomenon occurs between the vibration member and the moving member.
Conventionally, it is known that when an experiment regarding the motor is being carried out by varying the magnitude of the voltage applied to the ultrasonic-wave motor (the voltage is proportional to the size of the amplitude of the vibration member), there is a hysteresis between the voltage necessary at start time and the voltage when the moving member is stopped. When the inventors of the present invention examined the cause of this phenomenon, they discovered that it was caused by a contact force generated between the vibration member and the moving member as described above.
This phenomenon occurs when two objects having surfaces finished with a very high degree of precision are brought into contact with each other. If a slight amount of liquid such as condensed water is present on the surface where the two objects contact each other, then they are brought into close contact by a strong force due to the surface tension of the liquid. Since the surface where the rotor of the ultrasonic-wave motor contacts the stator thereof is machined to an extremely high level of surface precision by mirror finishing, and further since the rotor and the stator are brought into pressure-contact with each other by a spring, when a small number of water droplets is present between the surfaces of the two elements due to a temperature change, such a contact force is likely to occur. Water vapor present in a narrow clearance tends to condense as water. When such a contact force occurs, it becomes impossible to easily release the stator from contact with the rotor even if standing wave vibrations are generated in the stator at the start time.
This contact phenomenon may also occur suddenly when the number of rotations with respect to the moving member decreases sharply. Therefore, there is a risk that the motor will be suddenly stopped in a speed decreasing stage.
To start the moving member by overcoming a contact force when the contact force occurs between the vibration member and the moving member of the motor, it is necessary to apply a voltage even higher than that during the normal start-up in the conventional drive control method and apparatus. In such a structure, all the components including the circuit element acting as a component element of the drive control apparatus must be made pressure-resistant. As a result, the cost of the drive control apparatus is increased, and an apparatus having the motor installed therein becomes costly.